The present invention relates to a writable optical recording medium, comprising a light transmissive substrate and a recording film disposed thereon, particularly to an optical recording medium which has been improved on recording density (or packing density) to attain high density recording.
Hitherto, with respect to a so-called writable optical recording medium, there have been attempted and proposed various structures of such an optical recording medium which are capable of effecting high density recording so that the storage capacity (or memory capacity) thereof may be improved.
For example, in the conventional optical recording medium, there have been various attempts to improve memory capacity including; (1) a method wherein the distance between adjacent pits is reduced or the distance (or pitch) between adjacent tracks is reduced; (2) a method wherein plural substances each of which is responsive to each of many stimulations such as light and electricity are used as recording substances; (3) a so-called multiple wavelength optical recording medium comprising a plurality of colored matter films sequentially laminated on a substrate; (4) a PHB (Photo Chemical Burning) as a solid memory which has attracted much attention because it is capable of producing extremely high density recording; etc.
However, there is little possibility that the methods (2) to (4) as described above will be rapidly put into practice at the current level of the technique. Therefore, the above method (1) alone is an extension of the technique, and can be put into practice.
In the above method (1), however, recording (or writing) can be effected in a state where the distance between the adjacent pits is reduced or the pitch of tracks is reduced, but reading cannot be effected in such a state by use of a spot diameter of the present reading light (light beam). More specifically, when the present spot diameter of the recording light beam is used, it is possible to form a recording pit having a diameter which is smaller than that of the above spot diameter on the basis of a relation between the peak intensity of the light and the sensitivity of the recording film. However, in order to read such a recording pit without cross talk, it is necessary to further reduce the spot diameter of the present reading light. The spot diameter (D) is represented by a formula: D=.lambda. (wavelength)/NA (numerical aperture or aperture ratio of the lens). In order to reduce the spot diameter (D), it is necessary to increase the NA value and to decrease the .lambda. value. In order to increase the NA value, it is necessary to increase the aperture half angle. In such a case, however, it is necessary to use a certain working distance between the lens and the recording medium, and it is difficult to increase the present NA value. In addition, along with the development of SHG, THG, etc., it is theoretically possible to shorten the wavelength (.lambda.). However, such a method has not been put into practice yet.